This invention generally relates to optical printer heads, and particularly relates to micromachined optical microswitch printer heads which shine a lamp light through a plurality of addressable optical microswitches that let the light pass or block the light so as to generate light signals for graphic image formation.
Laser printers become popular due to a number of advantages over the rival inkjet technology. They produce much better quality black text documents than inkjets, and they tend to be designed more for the long haulxe2x80x94that is, they turn out more pages per month at a lower cost per page than inkjets. So, if it is an office workhorse that is required, the laser printer may be the best option. Another factor of importance to both the home and business user is the handling of envelopes, card and other non-regular media, where lasers once again have the edge over inkjets.
However, a laser source consists of a large relatively heavy, but delicate arrangement built into a large case. The case contains a single laser light source and a complex system of lenses and rotating mirrors that deflect the laser beam across the drum as it rotates. Complex timing is used to ensure that the laser can still produce a horizontal track across the drum surface while the drum continuously rotates. The edges of the drum are further from the laser than the center and so careful parallax correction must be employed. There is a limit to how fast the drum can be rotated while maintaining the horizontal scanning integrity.
LED (light-emitting diode) page printing is touted as the next big thing in laser printing. This technology produces the same results as conventional laser printing and uses the same fundamental method of applying toner to the paper The difference between the two technologies lies in the method of light distribution. The LED printer functions by means of an array of LEDs that create an image when shining down at 90 degrees. The advantage is that a row of LEDs is cheaper to make than a laser and mirror with lots of moving parts and, consequently, the technology presents a cheaper alternative to conventional laser printers. The LED system also has the benefit of being compact in relation to conventional lasers. Color devices have four rows of LEDsxe2x80x94one each for cyan, magenta, yellow and black tonersxe2x80x94allowing color print speeds the same as those for monochrome units.
The principal disadvantage of LED technology is that the quality of light from each element is dispersive and beam spot shapes are not uniform. The dispersed quality of light and the lack of uniformity of the beam spot shapes generate an uneven dot density of an output image such as an image containing black stripes. Moreover, an LED printer""s drum performs at its best in terms of efficiency and speed when continuous, high-volume printing is called for. In much the same way as a light bulb""s lifetime is shortened the more it is switched on and off, so an LED printer""s drum lifetime is shortened when used often for small print runs.
Along with developments in office automation products, the optical printers with improved performance are in strong demand.
It is therefore a general object of the present invention to provide an optical microswitch printer head that reduces the cost of printing pages and also to reduce the cost of making the printer.
A particular object of the present invention is to provide an optical microswitch printer head that enables the use of various light sources instead of only lasers and LEDs so as to extend usable optical spectrum range.
Another particular object of the present invention is to provide an optical microswitch printer head that enables the use of a high energy efficient light source instead of low energy efficient lasers and LEDs so as to reduce power consumption.
Still another particular object of the present invention is to provide an optical microswitch printer head that enables the use of a cheap light source instead of expensive lasers and LEDs so as to reduce production cost.
Still another particular object of the present invention is to provide an optical microswitch printer head that does not need to switch the light source for generating a light signal so as to increase the lifetime of the light source.
Still another object of the present invention is to provide an optical microswitch printer head in which formation of a pixel is accomplished through a micromachined optical switch so as to improve the resolution of the image.
Still another particular object of the present invention is to provide an optical microswitch printer head in which a needed driver circuit is integrated with the optical microswitches on a single substrate so as to simplify the control system and further reduce the production cost.
According to the features of the present invention, there is provided an optical microswitch printer head comprising an optical microswitch array with optical microswitches extending in a main scanning direction. The optical microswitch is based on a variable air gap Fabry-Perot cavity that is defined by two non-absorbing distributed Bragg reflectors. Since one of the distributed Bragg reflectors is supported by flexible beams, the length of the individual Fabry-Perot cavities can be set to be an odd or even multiple of a quarter wavelength of a working optical wave by applying a voltage. As a result, the optical microswitches can be pushed into a transmission state or xe2x80x9conxe2x80x9d state for letting a light passing through or a reflection state or xe2x80x9coffxe2x80x9d state for blocking the light.
In order to operate the optical microswitches the optical microswitch printer head includes a driver circuit. The driver circuit can be integrated in a single substrate with the optical microswitches or bonded onto a substrate that carries the optical microswitches.
The optical microswitch printer head can utilize a conventional gas discharge lamp as a light source. The light irradiated from the conventional gas discharge lamp shines over all the optical microswitches, but the optical microswitches are selectively switched xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d so as to generate light signals for graphic image formation.
The variable air gap Fabry-Perot cavity can be fabricated by surface micromachining technology. Surface micromachining adapts planar fabrication process steps known to the integrated circuit (IC) industry to manufacture micro-electro-mechanical or micro-mechanical system (MEMS) devices. The standard building-block processes for surface micromachining are deposition and photolithographic patterning of alternate layers of low-stress functional material such as a silicon nitride (Si3N4), amorphous silicon carbide (SiC) and polycrystalline silicon (also referred to a polysilicon) and a sacrificial material such as silicon dioxide (SiO2) or phosphorosilicate glass (PSG).
It is well-known that a low-stress functional material can be deposited by a low temperature process such as plasma enhanced deposition (PECVD). However, the etch selectivity of a conventional SiO2 sacrificial layer over a PECVD silicon nitride layer in hydrofluoric acid (HF) solution is very low. To solve this problem, an electrode material is inserted between the PECVD deposited silicon nitride layer and the SiO2 sacrificial layer. Such an electrode material comprises Indium Tin Oxide (In2O3:SnO2) or the like that does not be attacked by HF solution.
Surface micromachining results in a suspended mechanical structure generally consisting of a central plane and at least two side flexible beams. The two side flexible beams support the central plane and the central plane carries a distributed Bragg reflector thereon. Such a suspended mechanical structure can be moved with high precision with an applied voltage so as to change the length of the air gap between the two distributed Bragg reflectors. Since the entire process is based on standard IC fabrication technology, compact, highly, functional, and more self-contained micro-optic printer heads can be batch-fabricated.
The distributed Bragg reflectors comprise a stack of alternation layers of low refractive index material and high refractive index material. Such high refractive index materials include titanium dioxide (TiO2) with refractive index 2.34 and tantalum pentoxide2(Ta5O) with refractive index 2.16 at wavelength 400 nm. Such a low refractive index material includes SiO2 with refractive index 1.47 at wavelength 400 nm. The thickness of each layer is equal to xcex0/4n, where xcex0 is the light wavelength of the working light wave and n is the refractive index. A high quality distributed Bragg reflector is required to have high reflectivity and low absorption. It has been reported that at 1.55 xcexcm wavelength the reflectivity and stop band of a 5.5-period TiO2/SiO2 quarter-wavelength distributed Bragg reflector are 98.7% and 252 nm, respectively. A TiO2/SiO2 multilayered structure can be an alternative of the distributed Bragg reflectors. In addition, the refractive index of a SiNx layer can be adjusted up to 2.15 by an improved PECVD technology. Using such PECVD deposited SiNx, a 10 periods SiNx/SiO2 distributed Bragg reflector can have a stop band larger than 200 nm and a reflectivity higher than 99.7%.
In order to properly vary the length of the air gap of a Fabry-Perot cavity, the driver circuit is implemented such that the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d states of the variable air gap Fabry-Peroy cavities are set by two separate variable voltage sources. When one or more optical microswitches are switched to an xe2x80x9conxe2x80x9d state by applying one voltage source, the rest of the optical microswitches are switched to xe2x80x9coffxe2x80x9d state by applying the other voltage source. When a change in the working light wavelength takes place, the corresponding xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d voltage values also are changed.